1. Field of the Invention
The present invention relates to a charging circuit of a secondary battery, and more particularly to a charging circuit which detects a minute change of the voltage appearing in a waveform of the terminal voltage in the final stage of a process of charging the secondary battery, thereby to control the charging operation.
2. Description of the Related Art
Charging circuits for charging a secondary battery utilizing various control methods are known.
A charging circuit utilizing a first control method (hereinafter referred to as a first charging circuit) extracts minute voltage changes, i.e., components having a frequency higher than that of the terminal voltage change corresponding to the amount of charge, which appear in the waveform of the terminal voltage in the last stage of charging, thereby controlling the charging operation (Published Unexamined Japanese Patent Application (PUJPA) No. 3-118730). This charging circuit operates as follows. Minute voltage changes are extracted by a minute voltage change extracting circuit constituted by a differential circuit, and output signals from the minute voltage change extracting circuit are converted to pulse signals by a shaping circuit. Thereafter, the number of pulse signals is counted by a counter. When the count reaches a predetermined value, the charging of the secondary battery is controlled. The minute voltage change extracting circuit is constituted by an active filter using, for example, an operational amplifier.
A charging circuit utilizing a second control method (hereinafter referred to as a second charging circuit) has a differential circuit which is responsive to a change of the terminal voltage corresponding to the amount of charge of the secondary battery. When an output of the differential circuit becomes a preset value, the charging of the secondary battery is stopped (Published Examined Japanese Patent Application (PEJPA) No. 61-5339).
The first charging circuit is less influenced by the variation in charging voltage characteristics of a secondary battery due to the variation in the type of a battery, the charge current and the ambient temperature. Since the last stage of charging can be detected reliably for this reason, overcharge can be prevented. Further, even if a secondary battery using the first charging circuit, which has been charged, is recharged by mistake, overcharge can be prevented, since gas is generated slightly after the recharging is started, resulting in a minute voltage change.
In the second charging circuit, to charge a plurality of (n) secondary batteries connected in series, it is necessary to change the preset value in accordance with the number of the secondary batteries. Therefore, unless the charging voltage characteristics of the secondary batteries are the same, the characteristic of composite charging voltage becomes flat. For this reason, in the second charging circuit, the differential circuit cannot detect completion of a charging operation, and overcharge may occur. In contrast, when the first charging circuit is recharged, a composite of the minute voltage changes of the terminal voltage waveforms of the secondary batteries is superposed on a composite voltage waveform of the n terminal voltages of the secondary batteries. In this case, the minute voltage change of the secondary battery having the least electric capacitance appears first. Therefore, using the same charging circuit used to charge one secondary battery, the charging operation of the first charging circuit having a plurality of secondary batteries is controlled, when the secondary battery of the least capacitance is in the last stage of charging. Thus, overcharge is prevented.
Further, in the first charging circuit, the number of pulse signals obtained by shaping minute voltage changes is counted and a charging operation is continued until the count reaches a predetermined value. Therefore, even if a noise occurs, the charging is not controlled erroneously, although the count may slightly increase.
However, the first charging circuit requires a power source in addition to a power source for charging the secondary battery, resulting in a complicated structure. Therefore, the first charging circuit has a greater number of members and occupies a larger area. This may become a serious problem.
As described above, the minute voltage change extracting circuit is constituted by a differential circuit or a high-pass filter having an operational amplifier, a capacitor, and a resistor as its main components. Therefore, it takes time to stabilize the minute voltage change extracting circuit after the charging operation is started. More specifically, since the terminal voltage of a secondary battery changes greatly immediately after a charging operation is started, the minute voltage change extracting circuit is saturated and is not stabilized until the terminal voltage change is decreased. For this reason, when a secondary battery which has been charged is sent to the charging circuit for recharging by mistake, a minute voltage change which appears immediately after the start of recharging cannot be extracted at once. Thus, since the charging control delays, the secondary battery is accordingly overcharged, with the result that the lifetime thereof is reduced.
In a case where a commercial AC power source is used to recharge a secondary battery, a stable charging current free from noises included in a commercial AC current is required to accurately extract minute voltage changes which appear in the terminal voltage of the secondary battery. If a noise is included in a charging current, it may be extracted as a minute voltage change. To supply a charging current free from a noise, the following methods are known. One is to supply an output of a rectifier circuit, which is connected to a commercial AC power source, to the secondary battery through a constant current circuit using a dropper system. The other is to supply an output of a rectifier circuit to the secondary battery through a constant voltage circuit and a current limiting resistor.
However, in the former method, energy loss particularly in the constant current circuit is great. In the latter method, energy loss in the constant voltage circuit and the current limiting resistor is great. Therefore, in either method, the power supplying portion such as a transformer is large in size to cancel the energy loss. If a compact charging device is used, the temperature thereof increases greatly. Moreover, the entire cost of the charging circuit is increased owing to the constant current circuit or the constant voltage circuit.